Re: poor domU VBD performance.

[Jens Axboe <axboe@suse.de>]

On Thu, Mar 31 2005, Philip R Auld wrote:
> Rumor has it that on Thu, Mar 31, 2005 at 08:01:52PM +0200 Jens Axboe said:
> > On Thu, Mar 31 2005, Philip R Auld wrote:
> > > 
> > > My experience showed very little if any multipage 
> > > IO coming out of the front end.
> > 
> > There aren't that many users of multipage ios yet. direct io will use
> > it, ext2 will as well. iirc, -mm has patches for ext3 too. so it's
> > definitely improving :-)
> 
> Sorry, I was being sloppy with terminology :)
> 
> What I was getting at was that the backend  will split requests
> up and issue each physical segment as a separate bio  (at least in 
> the 2.0.5 tree I have in front of me). And that none of these 
> physical segments was more that 1 page. 
> 
> So the request merging in the back end OS is important, no?

I suppose it always is, since the merge criteria may have changed from
when the io was initially queued. If requests are always split into
single pages, then it becomes very important to merge at the backend.

-- 
Jens Axboe